The purpose of this project is to develop an animal model to assess the pathologic alterations and the hemodynamic dysfunction resulting from those alterations of cardiac bioprosthetic valves. Bioprosthetic valves implanted in juvenile sheep demonstrate the same pathologic alterations of degeneration and calcification as those implanted in humans, but these alterations occur in a much shorter period of time in valves implanted in sheep. Bioprosthetic valves of clinical quality from all commercial sources in this country have been implanted in the tricuspid and mitral positions of sheep to evaluate the pathogenesis of bioprosthetic valvular degeneration and to compare the characteristics and severity of these alterations in the different types of bioprosthetic valves.